Summary: In FY 2001 LMD continued to lead the WHO Collaborative Study of TgPVR mice and to coordinate results and analyses by participating consultants from CBER, NIBSC, and Canadian regulatory authorities. In June 1998 a WHO Consultation entitled Regulatory Model in Transgenic Mice (TgPVR 21) for Neurovirulence Tests of Oral Poliovirus Vaccine was convened by the WHO Global Programme for Vaccines and Immunization and hosted by LMD at CBER. Phase 4 of the WHO Collaborative Study with encreased number of tests of type 3 vaccine lots that had failed or passed the monkey neurovirulence test was completed in 1999. The WHO Consultation Group in June 1999 prepared documentation for presentation to the WHO ECBS. In October 1999 the WHO ECBS considered updated documentation and made the decision to include the Tg mouse neurovirulence test in the WHO Requirements for OPV as alternative to the monkey test for type 3 OPV. In framework of phase 5 of the WHO Collaborative Study additional experiments in transgenic mice were performed with type-1 and type-2 OPV preparations. The same statistical model developed for type-3 OPV was validated in several laboratories in FY'00. In June'00 the WHO Consultation Group met again to analyse and update documentation on type 1 and 2 OPV. Documentation prepared by the group was presented to the WHO Expert Committee on Biological Standardization in October 2000. The ECBS analysed updated documentation and made the decision to include the Tg mouse neurovirulence test in the WHO Requirements for OPV as alternative to the monkey test for all three types OPV. The implementation stage (introduction of the mouse test into manufacturing practice and by national regulatory authorities) is in progress. The WHO Consultation Group assigned LMD as a center for training of and consulting with all interested parties.